Sintering materials and attachment methods using same

ABSTRACT

Methods for die attachment of multichip and single components may involve printing a sintering paste on a substrate or on the back side of a die. Printing may involve stencil printing, screen printing, or a dispensing process. Paste may be printed on the back side of an entire wafer prior to dicing, or on the back side of an individual die. Sintering films may also be fabricated and transferred to a wafer, die or substrate. A post-sintering step may increase throughput.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 61/409,775 entitled “METHODS FOR DIEATTACHMENT OF MULTICHIP AND SINGLE COMPONENTS,” filed on Nov. 3, 2010,which is incorporated herein by reference in its entirety.

FIELD OF THE TECHNOLOGY

One or more aspects relate generally to methods for attachment ofvarious components, and more specifically to sintering materials andtechniques used for such attachment.

BACKGROUND OF THE INVENTION

Sintering has emerged as an alternative technology to conventionalsoldering. Sintering typically involves high temperature and highpressure processing to attach various components of an assembly.

BRIEF SUMMARY OF THE INVENTION

In accordance with one or more embodiments, a composition may include ametal powder having a d₅₀ range of about 0.001 to about 10 micrometers,the metal powder comprising about 30 to about 95 wt % of the paste, abinder having a softening point between about 50 and about 170° C., thebinder comprising about 0.1 to about 5 wt % of the paste, and a solventin an amount sufficient to dissolve at least the binder.

In some embodiments, the metal powder may comprise gold, palladium,silver, copper, aluminum, silver palladium alloy or gold palladiumalloy. The metal powder may comprise silver particles. In at least someembodiments, the metal powder may comprise nanoparticles. The metalpowder may comprise coated metal particles. In some embodiments, thecomposition may further include one or more functional additives.

In accordance with one or more embodiments, a film may comprise a layerof a composition including a metal powder having a d₅₀ range of about0.001 to about 10 micrometers, the metal powder comprising about 30 toabout 95 wt % of the paste, a binder having a softening point betweenabout 50 and about 170° C., the binder comprising about 0.1 to about 5wt % of the paste, and a solvent in an amount sufficient to dissolve atleast the binder. In some embodiments, the film may have a dry thicknessof about 5 to about 300 microns.

In some embodiments, the layer of the composition is on a polymeric,glass, metal or ceramic substrate. A polymeric substrate may comprisepolyester. A polymeric substrate may comprise a release coating.

In accordance with one or more embodiments, a method for producing afilm of metal particles may include applying a material comprising metalpowder having a d₅₀ range of about 0.001 to about 10 micrometers on asubstrate, and drying the material on the substrate to form the film.

In some embodiments, the substrate may comprise a polymeric substrate.The step of applying the material may comprise printing or casting thematerial. In at least some embodiments, the material may be printed in acontinuous layer. In other embodiments, the material may be printed toform an array of discrete shapes. In some embodiments, the method mayfurther comprise preparing the material.

In accordance with one or more embodiments, a lamination process forapplying a layer of metal particles to a component may include placingthe component on a film comprising the layer of metal particles on apolymeric substrate to form an assembly, applying heat to the assemblyin a range of about 50 to about 175° C., applying pressure to theassembly in a range of about 0.05 to about 3 MPa, and releasing thecomponent from the assembly, whereby the layer of metal particlesremains on the component and separates from the polymeric substrate. Insome embodiments, the film may be substantially the same size as thecomponent.

In accordance with one or more embodiments, a method for attachment mayinclude applying a film of metal particles to a substrate, placing a dieon the film to form an assembly, applying a pressure of less than about40 MPa to the assembly, and sintering the assembly at a temperature ofabout 175 to about 400° C. for about 0.25 seconds to about 30 minutes.

In some embodiments, a pressure of about 0.5 to about 20 MPa may beapplied. In at least some embodiments, a pressure of about 2.0 to about10 MPa may be applied.

In accordance with one or more embodiments, a method for attachment maycomprise applying a film of metal particles on a back side of a wafer,dicing the wafer to form a plurality of die, placing at least one die ona substrate to form an assembly, applying a pressure of less than about40 MPa to the assembly, and sintering the assembly at a temperature ofabout 175 to about 400° C. for about 0.25 seconds to about 30 minutes.In some embodiments, a pressure of about 2.0 to about 10 MPa may beapplied.

In accordance with one or more embodiments, a method for attachment maycomprise applying a film of metal particles on a back side of a die,placing the die on a substrate to form an assembly, applying a pressureof less than about 40 MPa to the assembly, and sintering the assembly ata temperature of about 175 to about 400° C. for about 0.25 seconds toabout 30 minutes. In some embodiments, a pressure of about 2.0 to about10 MPa may be applied.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments, are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. Where technicalfeatures in the figures, detailed description or any claim are followedby references signs, the reference signs have been included for the solepurpose of increasing the intelligibility of the figures anddescription. In the figures, each identical or nearly identicalcomponent that is illustrated in various figures is represented by alike numeral. For purposes of clarity, not every component may belabeled in every figure. In the figures:

FIGS. 1A-1D illustrate non-limiting examples of a formulation applied toa substrate in accordance with one or more embodiments;

FIG. 2 illustrates a stamping process in accordance with one or moreembodiments;

FIG. 3 illustrates a stamping support in accordance with one or moreembodiments;

FIG. 4 illustrates examples of stamping foils in accordance with one ormore embodiments;

FIG. 5 illustrates a die lamination process in accordance with one ormore embodiments;

FIG. 6 presents a schematic of a die attachment approach in accordancewith one or more embodiments;

FIGS. 7A and 7B illustrate non-limiting examples of depositiontechniques in accordance with one or more embodiments;

FIG. 8 illustrates a process of printing on a back side of a die inaccordance with one or more embodiments;

FIG. 9 illustrates an example of a die attached by the process of FIG. 8in accordance with one or more embodiments;

FIG. 10 illustrates a cross-sectional view of the attachment of FIG. 9in accordance with one or more embodiments;

FIG. 11 illustrates a process for transferring a film in accordance withone or more embodiments;

FIGS. 12A-12C present examples of films in accordance with one or moreembodiments;

FIG. 13 illustrates a lead frame printed with nanosilver paste inaccordance with one or more embodiments discussed in Example 1;

FIG. 14 presents a schematic of a process temperature profile inaccordance with one or more embodiments discussed in Example 1;

FIG. 15 illustrates a lead frame with attached dies in accordance withone or more embodiments discussed in Example 1;

FIG. 16 illustrates a cross-section of a join in accordance with one ormore embodiments discussed in Example 1;

FIG. 17 illustrates acoustic microscope image data in accordance withone or more embodiments discussed in Example 1;

FIGS. 18 and 19 illustrate die attached by processes in accordance withone or more embodiments discussed in Example 2;

FIGS. 20A-20C illustrate a lamination process in accordance with one ormore embodiments discussed in Example 3;

FIG. 21 illustrates die shear test data in accordance with one or moreembodiments as discussed in Example 4;

FIGS. 22A and 22B present data in accordance with one or moreembodiments discussed in Example 6; and

FIGS. 23A and 23B present data in accordance with one or moreembodiments discussed in Example 6.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one or more embodiments, a first component may bereliably attached to a second component. Some embodiments may involvesintering at a relatively low pressure and relatively low temperature incomparison to conventional sintering processes while achievingsubstantially the same results. In some embodiments, electroniccomponents may be attached. In other embodiments, non-electroniccomponents may be attached. High thermal and electrical conductivitybonds with high reliability and flexible bond line thickness may beprovided. One or more embodiments may promote a substantially uniformbond line thickness subsequent to sintering. Porosity, thermal andmechanical properties may also be substantially uniform. In somenon-limiting embodiments, a bond line having a thickness in a range ofabout 2 microns to about 100 microns may be achieved. Control anduniformity of bond line thickness over both large and small areas may beachieved. In some embodiments, loss of material during processing may bereduced which may result in overall cost savings of at least about 15%to about 20% in comparison to conventional attachment processes.Embodiments may also be associated with lower capital costs. Organicresidue may also beneficially be reduced such that post-cleaning may notbe necessary. In some embodiments, lead-free bonds are formed withoutintermetallics. Ease of processing may also be recognized in accordancewith one or more embodiments enabling high yield, high throughputmanufacturing processes. Various processes for the application ofpressure and heat may be enabled. Sintering materials and techniques inaccordance with one or more embodiments may find applicability in avariety of industries including hybrid electric vehicles, wind and solarpower generation including photovoltaic cells, transportation,industrial applications, consumer electronics and telecommunications.

In accordance with one or more embodiments, a sintering material may beused to facilitate attachment of various components. The sinteringmaterial may be used in a variety of forms. In some embodiments, asintering material may be used directly, for example, as an ink or apaste. In other embodiments, a sintering material may be used for theformation of a sintering film. A sintering film may be used in place ofa conventional sintering paste. In some embodiments, a sintering filmmay be produced by applying a fluid sintering formulation to a substrateand then drying the formulation on the substrate. The film may besubsequently transferred via a lamination process to facilitateattachment of components.

In accordance with one or more embodiments, a sintering material for usein any form may generally include a metal powder, a binder and asolvent. Various metals may be used depending on an intendedapplication. Properties of one or more components to be attached, or oneor more process conditions associated with an implemented attachmentprocess such as temperature and pressure, may impact selection of themetal powder for the formulation. Silver, gold, copper, palladium andaluminum are some non-limiting examples of metal powders that may beused. In some embodiments, mixtures of metals may also be used such assilver palladium alloy and gold palladium alloy. In some embodiments,metals, alloys or mixtures of metals and alloys may be used.

In accordance with one or more embodiments, the particle size of themetal powder may vary based on desired characteristics of theformulation for an intended application. In some embodiments, metalpowder with a d₅₀ range of from about 0.001 to about 100 micrometers maybe used. In certain embodiments, metal powder with a d₅₀ range of fromabout 0.001 to about 10 micrometers may be used. In one or moreembodiments, metal powder with a d₅₀ range of from about 0.001 to about0.01 micrometers may be used. In some embodiments, the particle size ofthe metal powder may be on the nanoscale. In some embodiments, theparticle size may be about 10 to about 100 nanometers. In still otherembodiments, the particle size may be about 10 to about 60 nanometers.In at least one non-limiting embodiment, the particle size may be about20 nanometers. In some non-limiting embodiments, nanosilver particlesmay be used.

In certain embodiments, the particles may be highly engineered. In someembodiments, the particles of the metal powder may be coated or capped.Some non-limiting examples of coatings include fatty acids, fatty aminesand starch. The amount of coating may constitute about 0.1 to about 20weight percent of the formulation. In some embodiments, the coating maypreferably constitute less than about 1 weight percent of theformulation. In at least some non-limiting embodiments, the coating mayconstitute about 0.5 to about 0.8 weight percent of the formulation. Inother embodiments, the particles of the metal powder may be uncoated oruncapped. Metal particles for the metal powder may be formed by varioustechniques. In at least one non-limiting embodiment, metal powder foruse in the formulation may be produced as described in U.S. Pat. No.7,968,008 to Parashar et al. which is hereby incorporated herein byreference in its entirety for all purposes. In some non-limitingembodiments, the metal powder may constitute about 30 to about 95 weightpercent of the formulation. In at least one specific embodiment, themetal powder may constitute about 80 weight percent of the formulation.

In accordance with one or more embodiments, a binder may providestrength and flexibility to the film subsequent to drying. The bindermay also provide adhesion of the formulation to a substrate on which itis deposited to facilitate film formation. The binder may yet alsoprovide adhesion of a film to a substrate to which it is transferred viaa lamination process. In some embodiments, various resins or rosins maybe used for the binder. Conditions and parameters associated withlamination and attachment processes to be used may impact binderselection. In some non-limiting embodiments, the binder may have asoftening point of between about 50° C. and about 170° C. In onenon-limiting embodiment, a binder with a softening point of about 90° C.may be used, such as Foralyn™ E partially hydrogenated rosin estercommercially available from Eastman. In at least one embodiment, thebinder may constitute about 0.5 to about 5 weight percent of theformulation.

In accordance with one or more embodiments, the formulation may alsoinclude one or more functional additives. Additives may generallyimprove adhesion to a substrate and the sintering behavior of thematerial. Non-limiting examples of additives include organic acids,amines, chlorinated or brominated diols or metallorganic compounds, suchas silver metallorganic compounds. Others may be commonly known to thoseskilled in the art. In some non-limiting embodiments, functionaladditives may constitute about 0.1 to about 2 weight percent of theformulation.

In accordance with one or more embodiments, various solvents may be usedto dissolve the binder and any additives present in the formulation.Various solvents such as but not limited to alcohols, diols, glycols orcombinations thereof may be used. In some embodiments, terpineol may bea preferred solvent. In other non-limiting embodiments, a mixtureterpineol and butyl carbitol may be used. In still other non-limitingembodiments, a mixture of terpineol, butyl carbitol and isopropanol maybe used. The amount of solvent present may vary depending on desiredproperties, such as viscosity, of the resulting formulation. Desirableviscosity may depend on intended use, such as a selected depositiontechnique. For example, a printing approach may require a lowerviscosity such that the formulation may exhibit one or morecharacteristics typical of an ink. Spraying may also be associated witha lower viscosity. Other deposition techniques, such as typecasting, maygenerally be associated with higher viscosity formulations. In someembodiments, higher viscosity formulations may exhibit one or morecharacteristics typical of a paste. In embodiments where it is desirableto form a sintering film from the formulation, viscosity may be adjustedaccordingly to facilitate that process. Loading of the metal powder,binder and/or additives with respect to the solvent system may generallybe adjusted to manipulate viscosity or another physical characteristic.In some non-limiting embodiments, the viscosity of the formulation maybe in a range of about 10 to about 200,000 cP. In at least one specificnon-limiting embodiment, the viscosity may be about 800 cP at 25° C.

In accordance with one or more embodiments, the formulation may beprepared by mixing the components. Various mixing devices and techniquesknown to those skilled in the art may be implemented, such as aplanetary mixer, orbital mixer or ultrasonic mixer. Milling may also beperformed in some embodiments, for example, so as to ensure a desiredtexture. In some embodiments, the formulation may be used directly in asintering process. For example, the formulation may be applied directlyto a component. In other embodiments, the formulation may be a precursorto a sintering film. A film may be used as a substitute for a paste in asintering assembly process.

In accordance with one or more embodiments, the formulation may beapplied to a backing layer or substrate and then dried to form a film.The substrate should generally provide adequate adhesion and support toa dried film for easy and reliable handling. The substrate may be rigidor flexible. The thickness of the substrate may vary. In somenon-limiting embodiments, the substrate may be between about 35 micronsand about 75 microns in thickness. The substrate should also release thefilm readily, such as during a lamination process. The substrate shouldgenerally not be affected by solvents in the formulation and should bestable at film drying and subsequent lamination temperatures. In someembodiments, the substrate may be a polymeric substrate. In at least onenon-limiting embodiment, the substrate may be a polyester substrate. Inother embodiments, a glass, metal, paper or ceramic substrate may beused. In some embodiments, the substrate may have a release layer orcoating. In some embodiments, materials such as silicone or aluminum maybe used as a substrate or a substrate coating. In at least onenon-limiting embodiment, the substrate may be a polyester film with asilicon release coating commercially available from Saint-Gobain. Insome embodiments, the substrate may be intended to facilitatetape-and-reel dispensing during an assembly operation.

In accordance with one or more embodiments, the formulation may beapplied to the substrate for film formation using various techniquesknown to those skilled in the art. In some embodiments, a printingtechnique may be used. Non-limiting examples of printing techniquesinclude inkjet, pad, screen, stencil, tape caster, gravure and offsetprinting. Other deposition methods may include recasting and spraytechniques. As discussed above, one or more physical properties of theformulation may be adjusted based on an intended deposition technique.Deposition of the formulation may be continuous over substantially anentire surface of the substrate or may be in discrete shapes withrespect to the substrate surface. In some non-limiting embodiments, theformulation may be applied on the substrate in shapes and/or sizescorresponding to dimensions of a component, such as a die, to beattached using the resulting film. Any geometry and any dimensions maybe implemented. Some non-limiting embodiments of the formulation appliedto a substrate are illustrated in FIGS. 1A-1D. In some non-limitingembodiments, rounds with a diameter of about 0.1 to about 500millimeters may be deposited. In other non-limiting embodiments,rectangles having a length or width of about 0.1 to about 500millimeters may be deposited on the substrate. In at least someembodiments, the formulation may be applied in a pattern on thesubstrate. In some non-limiting embodiments, the shapes and sizesapplied to a substrate may be intended to facilitate tape-and-reeldispensing during an assembly operation.

After deposition, the applied formulation may be dried on the substrate,such as in a batch or continuous oven. In some non-limiting embodiments,the deposited formulation may be dried at a temperature of around 130°C. for about 30 minutes. Dry thickness of the resulting films may varydepending on the deposition technique and the intended application. Drythickness may be from about 5 to about 1000 microns in accordance withone or more non-limiting embodiments. In some non-limiting embodiments,films may have a dry thickness of about 5 to about 300 microns. In someembodiments, films may be free standing. For example, films having a drythickness of from about 100 to about 300 microns may be removed from thesubstrate and used as free standing films for subsequent lamination andattachment in accordance with one or more embodiments. In otherembodiments, and in the case of relatively thinner films, the substratemay remain intact with respect to the film until removal during alamination process.

In accordance with one or more embodiments, a film may be used as asubstitute for a paste in a sintering assembly process. In someembodiments, using a film may enable different processes for applyingheat and pressure in an assembly process, such as in a sinteringprocess. In at least some embodiments, using a film may remove at leastone process step in an assembly operation. As discussed below, asintering film may be applied to a workside or on a substrate side tofacilitate assembly. With respect to the workside, a wafer may belaminated and then diced to form a plurality of laminated dies inaccordance with one or more embodiments. In other embodiments, a wafermay first be diced followed by lamination of individual die.

In accordance with one or more embodiments, a first step in anattachment process may be lamination of a film to either a component ora substrate. During lamination, a sintering film may be applied to adie, device, wafer, substrate, direct bonded copper (DBC), lead frame,metal disc or other element. A laminated component may subsequently beattached to a substrate. A laminated substrate may subsequently receiveone or more components. A film may be adhered to a backing layer asdescribed above with respect to film formation to facilitate lamination.In some embodiments, the film may be a blanket film much larger indimensions when compared to the element being laminated. In otherembodiments, the film may be generally patterned to be the substantiallythe same size as or smaller than the element being laminated. In atleast some embodiments, the film may contain one or more depositslocated in a specific pattern for use in a specific application that isthen transferred via lamination. Lamination may generally be performedvia a transfer or a stamping process. It is generally desired to avoidsintering of the film during lamination.

In a transfer approach, a film may be laminated to a substrate whichwill subsequently receive one or more components. During a transferapproach, a film may be placed on a substrate, for example, a directbonded copper (DBC) substrate, silicon wafer substrate, heat spreader orpiezoelectric substrate. The film may be pressed using a roller or otherappropriate equipment such as a press laminator to form an assembly.Heat and pressure may then be applied. Heat and pressure may be appliedsimultaneously or sequentially. In some non-limiting embodiments, theassembly may be subjected to a temperature of about 50° C. to about 175°C. In at least one non-limiting embodiment, a temperature of about 130°C. may be used. Heat may be applied using various known techniquesincluding infrared, induction, conduction, convection, radiation, andultrasonic. In some non-limiting embodiments, a heated placement tool orheated platen may be used. In some non-limiting embodiments, theassembly may be subjected to a pressure of about 0.05 to about 3 MPa. Inat least one embodiment, a pressure of about 0.2 to about 1 MPa may beused. The heat and pressure may generally be applied for a relativelyshort period of time, such as less than about a minute. In some specificembodiments, heat and pressure may be applied for about 10 to about 60seconds.

In a stamping approach, a film may be applied to a component, such as awafer or a die of various sizes. A stamping process is illustrated inFIG. 2. Equipment known to those skilled in the art such as a dieplacement machine may be used to facilitate lamination. In at least somenon-limiting embodiments, the film may be attached to a backside of thecomponent. Heat and pressure in the ranges noted above may then beapplied for a relatively short period of time. In one non-limitingembodiment, first and second platens may be heated to about 130° C. Apressure of about 1 MPa may be applied. Die size may impact a desiredforce applied. Dwell time may depend on the duration needed to generallyheat the assembly through. In some non-limiting embodiments, dwell timemay be about 3 minutes. In some non-limiting embodiments, dwell time maybe about 20 to about 100 milliseconds.

A support may be used to facilitate lamination during a stampingprocess. Rubber pads, etched foils, structures with cavities or othermaterials may be used for support. In some embodiments, the supportingstructure may create a cutting action to stamp out a portion of the filmfrom the backing layer. In some non-limiting embodiments, a stainlesssteel cavity etched foil may be used as a stamping support. The foil mayhave any desired thickness and any desired space between cavities thatmay ensure good repeatability and strength of the remaining film. FIG. 3presents the concept of the stamping support using a stamping foil as anexample. FIG. 4 presents examples of foils having 1 mm and 2 mm spacedcavities, respectively. In other embodiments, a rubber pad, such as asilicone rubber pad may be used as a stamping support. The thickness ofthe pad may vary and in some non-limiting embodiments may be about 3 mmthick. In still other embodiments, a hard substrate like epoxy orplastic or metal plate may be used for support. The support may alsofunction to protect the assembly and/or avoid adhesion to equipmentduring processing. The optimal tooling may depend on die area and otherfactors. For example, a rubber or foil support may be better for oneapplication than another. A system to hold the film down during stampingmay be used to facilitate the operation and may help protect the filmduring the operation.

A non-limiting example of a stamping process for die lamination isillustrated in FIG. 5 using a Datacon 2200 EVO die bonder. The bonderpicks up the die from the die holder or dicing tape. The die holdingtool is heated to about 130° C. The die is then placed on a silver filmwith about 50N force. As a result, a portion of the film havingdimensions substantially the same as those of the die is laminated onthe back side of the die. The laminated dies are then collected into awaffle pack for further attachment, such as via sintering, to a DBCsubstrate or lead frame.

In some embodiments, a laminated component may be baked to complete thelamination process. In some non-limiting embodiments, a laminatedcomponent may be baked for about one hour at about 130° C. Uponcompletion of the lamination process, for example the transfer orstamping process, the backing layer to which the film was adhered may beremoved. The laminated substrate or component may now include adeposited film of metal particles. In some specific embodiments, thefilm may be a film of nanometal particles. In at least one non-limitingembodiment, the film may be a film of nanosilver particles. As notedabove, the backing layer may alternatively be removed to produce afree-standing film prior to lamination in accordance with one or moreembodiments, such as those involving a relatively thicker film.

In accordance with one or more embodiments, a layer of sinteringmaterial, such as a film or paste, may be deposited on a die side. Inother embodiments, a layer of sintering material may be deposited on asubstrate side. In some non-limiting embodiments, the sintering film orpaste may include silver particles, such as silver nanoparticles. Ananosilver material, for example, may begin to sinter at temperaturesabove about 130° C. A sintering material may function to create a highlyreliable bond, for example, between a substrate and an element such as adie, device or other object. Pressure may be applied eithersimultaneously with heat, or prior to heating to the sinteringtemperature. If pressure is applied after heating, one or more benefitsof the sintering material, for example, low pressure sintering, fastsintering times, or the ability to form a reliable bond, may be lost. Inat least some embodiments, for multichip devices, chip placement andsintering may be performed in two different process steps. Applying thesintering material to the substrate may impose limitations on theprocess for single die packages and multichip packages. For example, thesubstrate should be kept below the sintering temperature before pressureand heat are applied. Rapid heating of the substrate may be desirablefor high speed production. Considering that the substrate may typicallybe the largest thermal mass, this may slow the process cycle time. Inaccordance with one or more embodiments, applying the sintering materialto the die side of the assembly may enable the substrate to be heated tosintering temperatures and may reduce process cycle times. Placement andsintering may be performed in one process step while the substrate is atsintering temperatures. Electronic and non-electronic components may beattached in accordance with one or more embodiments.

In accordance with one or more embodiments, a laminated component may bebonded or attached to a substrate. The laminated component may, forexample, be a die, device, wafer, or other element. The substrate may,for example, be a DBC, lead frame, metal disc, or other element. Duringbonding, the laminated component may generally be brought in contactwith the substrate to form an assembly. Heat and pressure may be appliedto the assembly for a sufficient duration to form a bond between thecomponent and the substrate. The bond should generally have one or moredesired characteristics such as those relating to strength, uniformityand bond line thickness. In some non-limiting embodiments, applied heatand pressure may be maintained for about 0.25 seconds to about 30minutes. Such durations may in some embodiments be associated withprocess or cycle times more than four times faster than conventionalsintering processes. A pressure of between about 0.5 MPa and 20 MPa maybe applied in one or more non-limiting embodiments. In some non-limitingembodiments, a sintering pressure of about 5 to about 10 MPa may beused. Such pressures may be as much as 25 times less than conventionalsintering techniques and may beneficially lessen stress on components,substrates and process equipment. A temperature of between about 175° C.and 400° C. may be applied in one or more non-limiting embodiments. Insome non-limiting embodiments, a sintering temperature of about 230° C.to about 260° C. may be used. In some embodiments, the heat may beapplied by heating a placement tool, weight, spring, or mass used forplacing, holding, or placing and holding the component. In otherembodiments, heat may be applied via a continuous or batch oven, or byheating a platen located beneath the substrate or above the component.In some embodiments, both the placement tool and a platen or multipleplatens located above and/or below the assembly may be heated. Heat maybe applied via infrared, induction, conduction, convection, radiation,ultrasonic, or other technique. Multiple laminated components may bebonded to a single substrate or multiple substrates in a serial or aparallel approach. In at least one non-limiting embodiment, sinteringmay be performed at a temperature of about 200° C. for about 15 minutes.

In accordance with one or more embodiments, a component may be bonded orattached to a laminated substrate. The component may, for example, be adie, device, wafer, or other element. The laminated substrate may, forexample, be a DBC, lead frame, metal disc, or other element. Duringbonding, the component may generally be brought in contact with thelaminated substrate to form an assembly. Heat and pressure may beapplied to the assembly for a sufficient duration to form a bond betweenthe component and the substrate. The bond should generally have one ormore desired characteristics such as those relating to strength,uniformity and bond line thickness. In some non-limiting embodiments,applied heat and pressure may be maintained for about 0.25 seconds toabout 30 minutes. A pressure of between about 0.5 MPa and 20 MPa may beapplied in one or more non-limiting embodiments. In some non-limitingembodiments, a sintering pressure of about 5 to about 10 MPa may beused. Such pressures may be as much as 25 times less than conventionalsintering techniques and may beneficially lessen stress on components,substrates and process equipment. A sintering temperature of betweenabout 175° C. and 400° C. may be applied in one or more non-limitingembodiments. In some non-limiting embodiments, a sintering temperatureof about 230° C. to about 260° C. may be used. In some embodiments, theheat may be applied by heating a placement tool, weight, spring, or massused for placing, holding, or placing and holding the component. Inother embodiments, heat may be applied via a continuous or batch oven,or by heating a platen located beneath the substrate or above thecomponent. In some embodiments, both the placement tool and a platen ormultiple platens located above and/or below the assembly may be heated.Heat may be applied via infrared, induction, conduction, convection,radiation, ultrasonic, or other technique. Multiple components may bebonded to a single laminated substrate or multiple laminated substratesin a serial or a parallel approach. In at least one non-limitingembodiment, sintering may be performed at a temperature of about 200° C.for about 15 minutes.

In some embodiments, equipment for attachment of multiple components maybe a hydraulic or pneumatic press, such as one manufactured by CarverInc. A typical press may have a large heating platen to accommodate alarge number of substrates. The platen may provide heat of about 200 toabout 300° C. and the press may be capable of providing force sufficientto generate a pressure of about 1 to about 20 MPa on attachedcomponents. An example of one such press is the Carver MH 3891 press.For attachment of a single die or component, equipment such as an ESECsoft solder die bonder SSI 2009 may be used. The bonder may be capableof applying a bonding force of about 100N and heat up to about 400° C.

In accordance with one or more embodiments, the sintering process mayconvert the metal particles in the sintering material to bulk metal.Without wishing to be bound by any particular theory, upon onset of thesintering process, nanoparticles may convert to micron particles andthen convert to bulk metal through grain growth and densification astemperature and time increases, even without applied pressure. A densemetal film may be formed with strength comparable to bulk metal.

In accordance with one or more embodiments, following any of thesintering processes described above, assembled parts may be post-treatedin the oven, for example, at about 300° C. for about 5 to 10 minutes.Such post-sintering may result in improved strength of assembly joints.Use of post-sintering may also minimize overall process sintering timeand increase throughput of the sintering press.

In accordance with one or more embodiments, resulting bonds may beassociated with high thermal and electrical conductivity. Non-limitingexamples of silver bond lines may have a thermal conductivity in therange of approximately 250 W/m° K. Some non-limiting examples of silverbond lines may have a density of about 85 to about 95% of bulk silver.The bonds may also be associated with high thermal shock resistancewhich may contribute to extended die bond life. In some embodiments,bonds may exhibit more than 40 MPa bond strength (die shear) over morethan 2000 cycles at 220° C. In at least some embodiments, Nodelamination may occur even after 800 thermal shock cycles at 220° C.

In accordance with one or more embodiments, silver may be appropriatefor high temperature packing applications because of its high electricaland thermal conductivity, low susceptibility to oxidation, and meltingpoint that is sufficient to withstand high operating temperatures. Insome embodiments, a silver bond may be more than five times morereliable than solder.

In accordance with one or more embodiments, sintering materials andtechniques may be useful in the attachment of Si, SiC, GaN or othersemiconductor devices.

In accordance with one or more non-limiting embodiments, components suchas semiconductor devices may be attached to a substrate using a metalpaste rather than a film. FIG. 6 presents a schematic of onenon-limiting approach to die attachment using low temperature and lowpressure. In the process, a metal paste may be printed on a substrate.Various pastes may be used. In some non-limiting embodiments, the metalpaste may be a nanometal paste such as a nanosilver paste. In onenon-limiting example, a silver paste commercially available from AlphaMetals Inc., such as one containing silver nanopowder and an organicvehicle, may be used. Various substrates may be used, such as barecopper lead frames, or copper lead frames including a silver or goldcoating. Ceramic substrates and DBC may also be used. Various dies, suchas those involving silicon, silicon carbide or any other chips ordevices may be used.

In accordance with one or more embodiments, a die attach process mayinvolve printing on a substrate, such as on a lead frame. In suchembodiments, printing paste on the substrate may be achieved by varioustechniques including stencil/screen printing or by dispensing. Thesubstrate may be any desired material, such as a copper based materialor a metallized ceramic, for example, DBC. The attachment processillustrated in FIG. 6 may include non-limiting steps of printing pasteon a substrate, drying the paste, for example at 130° C., placing a dieon the printed paste, placing the die-substrate assembly on a heatingstage, applying pressure, raising the temperature to about 250° C. toabout 300° C., and holding the pressure and temperature for about 30 toabout 90 seconds.

In accordance with one or more embodiments, a die attach process may usestandard soft solder die bonder equipment. A pick-up tool may pick a diefrom a dicing tape and place it with a force onto a heated substrate.Paste, such as silver paste, may be printed either on the substrate oron the back side of an individual die, entire wafer or applied as afilm. The deposition may be performed by print or applied as a film vialamination. Examples of non-limiting processes in accordance with one ormore embodiments are shown schematically in FIGS. 7A and 7B. FIG. 7Aillustrates printing on the substrate while FIG. 7B illustrates printingon the backside of the component.

In accordance with one or more embodiments, a die attach process mayinvolve printing on a substrate via dispensing printing. Silver pastesuch as a nanosilver paste may be dispensed on a lead frame and thenattachment may be performed in a manner similar to that described above.Dispensing techniques may not produce a substantially flat surfacecomparable to that produced by stencil or screen printing. Various typesof dispensing equipment are available for industrial and laboratory use.

In accordance with one or more embodiments, a die attach process mayinvolve printing on the back side of a die, such as a wafer. Inaccordance with one or more embodiments, paste, such as nanosilver pastemay be applied the back side of a wafer in various ways. In someembodiments, nanosilver paste may be applied to the back side of anentire wafer and then the wafer may be diced into individual chips. Inother embodiments, the wafer may be diced first and then paste may beapplied to the back side of individual chips.

In accordance with one or more embodiments, paste may be applied on theback side of an entire wafer. The paste may be dried after application.In some non-limiting embodiments, the paste may be dried at about 130°C. for about 30 to 90 minutes. A reinforcing solution may be applied,such as by spraying or spin coating. The wafer may then be placed on adicing tape and the wafer may be diced. The diced wafer on the tape maybe introduced to a soft solder die bonder. Individual die may bepicked-up and placed on a substrate with force, such as that sufficientto produce a pressure of about 5 to 10 MPa. Heat, such as at atemperature of about 250 to 400° C. may be applied. The pressure may beheld, such as for sintering, for about 0.5 to 1 second. Post-sinteringmay also be performed, such as at a temperature of about 250 to 300° C.for about 10 to 30 minutes.

In accordance with one or more embodiments, an important factor in theattachment process is the ability of a printed paste layer to withstanddicing and a pick-up process without damage. Without sintering, aprinted silver layer may be marginally strong and its adhesion to theback side of a wafer may be weak. Without proper strength, a silverlayer may be destroyed by dicing and/or during a die pick-up step. Toreinforce a silver layer after printing and drying, a solutioncontaining a polymer or resin may be sprayed or spin coated over thesliver layer. After drying, this overcoat may ensure silver layerstrength and adhesion to the wafer. It is desirable that the polymerand/or resin will decompose during a later sintering step in order tominimize the effect of any residue on the properties of the sinteredsilver layer. Non-limiting examples of polymers and resins which may beused include PMMA, PVP, Joncryl 682, and hydrogenated rosins. In someembodiments, certain resins, such as hydrogenated rosins or likematerials, may be incorporated into the composition of the silver paste.Application of the reinforcing solution may be optional, whether or notsuch materials are incorporated into the paste formulation.

In accordance with one or more embodiments, silver may be printed in theform of bumps rather than as a continuous film. Bumping isconventionally used with various devices such as semiconductor chipsincluding memory or processors. Applied bumps are typically in the rangeof about 80-150 microns in diameter and made of solder. In accordancewith one or more embodiments, solder may be replaced with silver forhigh thermal conductivity and heat dissipation.

In accordance with one or more embodiments, paste may be printed on theback side of individual dies. One non-limiting embodiment of thisprocess is schematically shown in FIG. 8. The dies may be picked fromdicing tape and placed in a stencil-holder. The thickness of the stencilholder may generally be equal to the thickness of the die plus theprinting thickness. The holder may be flipped to expose the back side ofthe dies. Nanosilver paste may then be printed on the back side. Thepaste may be dried, for example at 130° C. for about 30 minutes. Thestencil-holder may then be flipped to expose the top side of the dies.The dies may then be individually picked up and placed on a substrate.The substrate may be preheated, such as up to a temperature of about400° C. In some embodiments, the dies may be placed with a forcesufficient to produce a pressure of about 5 to 20 MPa. In at least oneembodiment, the pressure may be held for about 0.5-2 sec. FIG. 9presents an example of a die attached by the process of FIG. 8. FIG. 10presents a cross-sectional view of the attachment, indicating a fullysintered silver layer connected to the metallization on the die and themetallic substrate.

In accordance with one or more embodiments, films may be fabricated andtransferred to a wafer, die or substrate. Nanosilver films may befabricated using specially formulated nanosilver inks, pastes ordispersions. Such formulations may include nanosilver powder, solventsand binders. Films may be made by depositing the formulations on asubstrate and drying the formulation at room or elevated temperature.Typical substrates may involve polymer, mylar, paper and aluminum foils.The films may be deposited on the substrate using printing, doctor bladeor spraying. The films may be continuous and/or patterned to a desiredgeometry. The films may be deposited on a flexible or rigid carrier. Theprinted films may typically be dried in an oven, such as at about 70° C.to 130° C. for about 10 to 40 minutes. The carrier may then be removedand free standing films may be created. Fabricated films may betransferred to a wafer, die or a substrate using a transfer process byapplying heat and pressure. Applied pressure may typically be in therange of about 0 to 2 MPa or higher, and applied temperature may be inthe range of room temperature to about 150° C. The wafer, die orsubstrate may then be attached using any known sintering technique,including the attachment processes described above. A non-limitingexample of a process for transferring a film onto a wafer isschematically shown in FIG. 11. FIGS. 12A and 12B present examples ofprinted films and FIG. 12C illustrates free-standing nanofilms.

A film may be transferred onto an individual die, component or heatspreader under the process conditions described above. A component maybe introduced to a continuous or patterned film via a common pick andplace tool. The film may adhere to the back side of the component whichmay then be introduced to a final sintering process. In the case of acontinuous film, the portion of the film that will be transferred willgenerally equal the dimension of the component. For sintering multipledies simultaneously, the dies or components containing applied film maybe temporarily tacked to a substrate and then sintered by any methoddescribed above. Nanofilms may be formed using any nanometal powder.Nanofilms may include various functional additives to improve desiredphysical and/or mechanical properties and may be considered as“composite” nanofilms.

In accordance with one or more embodiments, nanosilver and/or othermetals may be used for die attach. A continuous film of paste may bestencil or screen printed. Spin coating, spray coating, doctor bladingor casting may also be used. In some embodiments, silver may be printedin the form of bumps, such as 50-200 microns in size. The film or bumpsmay be transferred. A film of nanosilver paste may be applied to anentire wafer prior to dicing. A film may be applied as a step in dicing,or applied after wafer fabrication is completed. Paste and particles maybe formulated with a specific composition and properties so as topromote adhesion to the wafer, adhesion to the dicing film, and cohesionbetween particles. Formulations may also facilitate drying and storage,as well as not inhibit the sintering process and bonding processrequired for application. In order to facilitate dicing, pressure may beapplied to the printed nanosilver layer to enhance cohesion and adhesionto the wafer. Dicing may be enabled using stencils to produce dicingkerfs for saw blades, as well as by reinforcement of die adhesion todicing tape during dicing. Sintering may be enabled using any form ofheat including convection, radiant, induction and microwave. Rapidsintering, such as less than one minute, may be enabled. Slow sintering,as well as sintering combined with diffusion may also be used.Application of silver paste on the die side may provide little or notemperature limitation from the rubber side, as well as little or notemperature limitation from substrate side. Application of silver pasteon the die side may also enable attachment done by a hard tool and maygenerally provide a broader process window. In some embodiments, waferto wafer bonding may be performed, as well as other bonding applicationsincluding wire bonding, ribbon bonding, hermetic sealing, lld sealing,metal to metal bonding, metal to glass bonding, general bonding andbonding to various polymeric materials.

The function and advantages of these and other embodiments will be morefully understood from the following examples. The examples are intendedto be illustrative in nature and are not to be considered as limitingthe scope of the embodiments discussed herein.

Example 1

FIG. 13 illustrates an example of a lead frame printed with nanosilverpaste using a 100 micron thick stencil. The stencil thickness maygenerally dictate the bond line thickness. After printing, the leadframe was dried in the oven at 130° C. for 30 minutes. The equipmentused to demonstrate the process was a soft solder die bondercommercially available from ESEC (Switzerland). The standard equipmentwas modified to provide a heating option on the pick-up arm. FIG. 14presents the temperature settings that were used in various zones inwhich the temperature of the lead-frame was kept below 150° C. Thetemperature in heating zones 1 through 6 was set below 150° C., in ordernot to overheat and pre-sinter the paste. The temperature in zone 7 inwhich the attachment process took place was set to about 300° C. toabout 400° C. and zone 8 was set to the same temperature. The printedlead frames were loaded into the machine which indexed them through theheat zones with a speed to provide a bonding time between 0.5 and 1second. FIG. 15 illustrates a lead frame with the attached dies. Afterattachment of the dies in the soft solder bonder, some of the lead framewas heat treated (post-sintered) in the oven at 300° C. for about 10minutes to increase die adhesion to the lead frame. The typical dieshear force was about 20 MPa.

FIG. 16 illustrates a typical cross section of the created joint.Reliability of the joint was tested in a liquid-to liquid thermal shocktest. Temperature settings were −50° C. to +125° C. with a 6 minutecycle time. Acoustic microscope images indicated no or only minorchanges in the morphology of the joints indicating good reliableconnection of the dies to the lead frame as illustrated in FIG. 17.

Example 2

In accordance with one or more embodiments, the process for dieattachment following dispensing may vary depending on the die size andequipment. In a first process, paste may be dispensed and then leveledusing a nonstick surface, such as a Teflon® pad. The paste may then bedried, for example at about 130° C. for about 30 minutes. The die maythen be placed and sintered, such as at a temperature of about 250° C.to 300° C. In a second process, paste may be dispensed and a die may beplaced on the wet surface with minimum force. The paste may then bedried, for example, at about 130° C. for about 20 to 30 minutes. The diemay then be placed and sintered at about 250° C. to 300° C. FIG. 18illustrates a die attached by this second process. In a third process,paste may be dispensed and then partially dried so as to keep the pastesoft. In some non-limiting embodiments, the partial drying may be atabout 70° C. for about 5 minutes. The die may then be placed andsintered at about 250° C. to 300° C. following the partial drying. FIG.19 presents a die attached by this third process.

Example 3

A wafer lamination process was demonstrated. A round silicone wafer withsilver backside metallization was placed on an aluminum plate. A sheetof sintering film was placed on the wafer and a silicone rubber pad wasplaced on the sintering film. The silicone rubber pad was then coveredwith Teflon foil. The resulting assembly was placed on betweenpre-heated platens (130° C.) and a pressure of about 1 MPa was appliedfor about 3 minutes. The wafer and the film after lamination areillustrated in FIG. 20A. The sheet of sintering film was then removedfrom the wafer as illustrated in FIG. 20B. A round portion of the filmwas laminated to the silicone wafer thus exposing a portion of thebacking layer on the sheet, and the rest of the sintering film remainedon the backing layer. The laminated wafer is illustrated in FIG. 20C.The laminated wafer was then baked for about one hour at about 130° C.

Example 4

Die were sintered to a substrate using the same sintering material inboth paste and film forms in accordance with one or more embodiments.The process conditions for sintering with both the paste and film wereabout 250° C. at 10 MPa in air. Data was collected for sintering timesof about 40 seconds, 60 seconds and 80 seconds. Die shear tests wereperformed for the resulting bonds formed using both the paste and thefilm. The results are illustrated in FIG. 21 and reflect comparableresults for both formats.

Example 5

A pick and stamp process using a sintering film in accordance with oneor more embodiments was demonstrated for both small dies and large diesusing various combinations of the following equipment parameters:

Equipment Variables Dies  5 × 5 12 × 12 Backing Foil Thin (35 μm) Thick(75 μm) Stamping support Stainless Steel cavity foil  80 microns thick120 microns thick 180 microns thick Silicon Rubber PCB Substrate

Tests with an applied force of from about 10N to about 50N wereconducted. Delay between applications of pressure by the nozzle rangedfrom 50 ms to 1000 ms. Tests with an applied temperature of from about130° C. to about 160° C. were conducted.

For small dies, best results were obtained with the thin backing foiland using a PCB substrate as a stamping support. For large dies, bestresults were obtained with the thick backing foil and using a 120microns thick stainless steel cavity foil as a stamping support. Optimaloperating parameters for small dies were a force of about 2500 grams, adelay of about 500 ms and a nozzle temperature of about 145° C. Optimaloperating parameters for both large dies were a force of about 2500grams, a delay of about 1000 ms and a nozzle temperature of about 150°C. 1 mm minimal distance between two dies was achieved for both smalland large dies.

Example 6

Dies were attached to gold and DBC substrates with sintering films inaccordance with one or more embodiments. Images before a bend test areillustrated in FIG. 22A and images after the bend test are illustratedin FIG. 22B. The bend test showed no detachment of the die from the goldand DBC surfaces. FIG. 23A presents CSAM images before thermal shock.FIG. 23B presents acoustic microscope images after 500 cycles of liquidto liquid thermal shock from −50° C. to 165° C. No delamination or bonddegradation was demonstrated indicating bond integrity.

It is to be appreciated that embodiments of the methods and apparatusesdiscussed herein are not limited in application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Themethods and apparatuses are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Examples of specific implementations are provided herein forillustrative purposes only and are not intended to be limiting. Inparticular, acts, elements and features discussed in connection with anyone or more embodiments are not intended to be excluded from a similarrole in any other embodiment.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. Any references toembodiments or elements or acts of the systems and methods hereinreferred to in the singular may also embrace embodiments including aplurality of these elements, and any references in plural to anyembodiment or element or act herein may also embrace embodimentsincluding only a single element. The use herein of “including,”“comprising,” “having,” “containing,” “involving,” and variationsthereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.Any references to front and back, left and right, top and bottom, upperand lower, and vertical and horizontal are intended for convenience ofdescription, not to limit the present systems and methods or theircomponents to any one positional or spatial orientation.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention.Accordingly, the foregoing description and drawings are by way ofexample only.

What is claimed is:
 1. A sheet of sintering film, comprising: asubstrate; a release coating disposed on the substrate; and a layer of adry sintering material having a thickness of about 5 to about 300microns disposed on the release coating on the substrate; wherein thesintering material comprises, prior to drying: (i) a metal powder havinga d₅₀ range of about 0.001 to about 10 micrometers, the metal powdercomprising about 30 to about 95 wt % of the sintering material; (ii) abinder having a softening point between about 50 and about 170° C., thebinder comprising about 0.1 to about 5 wt % of the sintering material;and (iii) a solvent in an amount sufficient to dissolve at least thebinder; wherein the substrate is configured to be removed during alamination process; and wherein the layer of sintering material isapplied in discrete shapes with respect to a surface of the substrate,and the discrete shapes have geometries corresponding to dimensions of acomponent to be attached using the layer of sintering material.
 2. Thesheet of sintering film of claim 1, wherein the substrate is apolymeric, glass, metal, or ceramic substrate.
 3. The sheet of sinteringfilm of claim 1, wherein the sintering material further comprises one ormore functional additives.
 4. The sheet of sintering film of claim 3,wherein the one or more functional additives comprise chlorinated diolsand/or metallorganic compounds.
 5. The sheet of sintering film of claim1, wherein the metal powder comprises gold, palladium, silver, copper,aluminum, silver palladium alloy, or gold palladium alloy.
 6. The sheetof sintering film of claim 5, wherein the metal powder comprises silverparticles.
 7. The sheet of sintering film of claim 5, wherein the metalpowder comprises nanoparticles.
 8. The sheet of sintering film of claim5, wherein the metal powder comprises coated metal particles.
 9. Thesheet of sintering film of claim 1, configured to provide a bond linecharacterized by a thermal conductivity of about 250 W/m° K.
 10. Thesheet of sintering film of claim 1, configured to provide a bond linecharacterized by a bond strength of at least about 40 MPa.
 11. The sheetof sintering film of claim 1, configured for a sintering operation at atemperature of about 175° C. to about 400° C.
 12. The sheet of sinteringfilm of claim 11, configured for a sintering operation at a temperatureof about 230° C. to about 260° C.
 13. The sheet of sintering film ofclaim 1, configured for a sintering operation at a pressure of betweenabout 0.5 MPa and 20 MPa.
 14. The sheet of sintering film of claim 13,configured for a sintering operation at a pressure of between about 5MPa and 10 MPa.
 15. The sheet of sintering film of claim 1, configuredto provide a bond line having a thickness in a range of about 2 micronsto about 100 microns.
 16. The sheet of sintering film of claim 1,wherein the discrete shapes are constructed and arranged to facilitatetape-and-reel dispensing of the film during an assembly operation.